Method and apparatus for the sterilization of biological waste

ABSTRACT

Continuous biowaste sterilization process, the operability of which is ensured in such a way that during process startup, extreme conditions possibly coming into question during operation are simulated, i.e. the greatest possible flow rate and the lowest temperature of the feed flow. In the startup stage, a liquid to be sterlized is circulated in the apparatus, until the capacity has been ascertained, whereafter the discharge stream can be conducted to a sewer system. The apparatus according to the invention comprises, in the flow direction of the biowaste-containing liquid, a storage tank, at least one feed pump capable of delivering a constant flow, at least one heating unit, at least one cooling unit, and a circulation circuit for circulating the biowaste-containing liquid through the heating unit, as well as connecting piping and valves. By means of specific piping and valve arrangements it is also ensured that all parts of the apparatus can be sterilized for maintenance purposes.

The invention relates to a continuous sterilization process forbiological waste and an apparatus for applying said process, the mainline of which comprising, in the flow direction of a biowaste-containingliquid, a storage tank, at least one feed pump, at least one heatingunit, at least one cooling unit and a circulation circuit forcirculating the biowaste-containing liquid through the heating unit, aswell as appropriate piping and valves.

Biological waste is produced e.g. in hospitals, agricultural orbiological research and production facilities, plasma fractionationfacilities, etc. Biological wastes produced in such facilities cannot bedirectly conducted to a sewer system, as these wastes often containmicro-organisms, such as bacteria, viruses, germs and the like, whichare hazardous to humans and animals. Prior to conducting to a sewersystem, such biowaste must first be deactivated in a treatment plantdesigned for this purpose. For the treatment of biowaste, differenttreatment plants have been designed in which biowaste is sterilizedprior to conducting to the sewer system. The sterilization of biowastecan be carried out chemically or by means of heat. The treatment plantscan operate continuously or batchwise.

In publication DE 40 16 116, a process is disclosed for the continuouspurification of waste waters laden by micro-organisms, such as bacteria,viruses, germs and the like. In said process the waste water is heatedto a disinfection temperature by means of a heat exchanger and/or adirectly injected hot steam, whereafter the waste water is conducted toa dwell circuit, e.g. to a pressurized vessel. In said dwell circuit,the wastewater is held at the disinfection temperature for apredetermined time. The residence time is measured by injecting anindicator into the wastewater at regular intervals, e.g. every fiveminutes for 5 seconds, and by measuring the presence of this indicatorin the outlet of the dwell circuit. The time difference betweenindicator injection and detection shows the actual residence time, whichcan be compared with the residence time setting.

In the journal Pharmaceutical Engineering, May/June 2001, pages 70 to82, an article “Biowaste Systems” by Carl J. Carlson appears, relatingto facilities for the treatment of biowaste. The article deals withbiowaste treatment facilities of different type as well as withdimensioning principles and problems relating thereto.

According to said article, a typical thermal continuous biowastesterilisation apparatus comprises a separating unit for solid matter, astorage tank, a heating unit and a dwell circuit as well as acirculation circuit for circulating biowaste through said heating unitand said dwell circuit. According to the article, a typical continuousapparatus comprises the following stages: a heating stage, wherebybiowaste is circulated in a heat exchanger and in a dwell circuit, untila temperature sufficient to kill the micro-organisms is reached. This isfollowed by an operating stage when the biowaste has reached therequired temperature over the whole length of the heat exchanger.Thereby the treated biowaste is conducted through cooling equipment to asewer system. If one or several sterilization parameters (temperature inthe dwell circuit, pressure etc.) go outside the predetermined value,and the biowaste is therefore insufficiently sterilized, the processenters a hold state, where the biowaste is circulated through theheating unit and the dwell circuit until the parameter or parameters inquestion are again within the given limits. In case of an alarm, theapparatus enters the cooling mode, in which the operation of the heatingunit is stopped, and the biowaste is recirculated back to the pump feedline until the apparatus is again in working order. According to saidarticle, provisions for the steam sterilization of the parts downstreamfrom the storage tank should be provided, as well as provisions forpreventing the transfer of the active biowaste to the cooling circuit.In addition, steam sterilization of the storage tanks the piping,venting filters, etc. should be provided in the apparatus.

The present invention is characterized by the features presented in thecharacterizing parts of claims 1 and 3.

In the starting up stage of the present heat sterilization process forbiowaste, conditions at the upper limits of the capacity thereof, i.e.“worst case” conditions are used. Thereby a liquid is conducted througha heating unit at maximum flow rate, which liquid is subsequently cooledto a level, which corresponds to the lowest defined temperature of thefeed flow. When a sufficient sterilization capacity has been reachedunder these extreme conditions, in other words, when the temperature inthe sterilization zone can be maintained at such a high level that itcorresponds to the residence time required at the flow rate in question,the process operates with great reliability under all conditionsoccurring during operation. Preferably, the invention further comprisesan arrangement by means of which the tightness of those valves, whichare critical during startup and in exceptional situations, can beensured, and, if necessary, the whole valve system can be sterilized formaintenance measures.

In a process according to the invention, during startup the operabilityof the process is thus ensured, that at maximum capacity of the pump aliquid flow is conducted through the heating unit, the temperature ofwhich flow has been lowered in the return circuit to a levelcorresponding to the minimum temperature of the water in the storagetank during operation. The sterilization stage following the heatingstage comprises at least measurement of the temperature at the outletend. When the residence time of the liquid in the sterilization zone hasbeen found sufficient under the above conditions, it can also be assumedthat the operability of the process can be maintained independently ofvariations occurring in the feed.

A device according to the invention is provided with means for verifyingthe sterilization ability at maxim load during startup. For verifyingthe capacity of the device, the lowest water temperature and thegreatest possible flow are used. In order to ensure that the testactually represents the worst case, the maximum flow must be limited so,that during actual operation it cannot exceed the flow used in the test.Preferably, this is achieved using a positive displacement pump, whichat a constant speed of rotation (determined by the electrical motorused) always delivers a constant flow, independent of the pressures atthe suction and discharge ends. When a centrifugal pump is used, themaximum flow is limited by setting standards for the suction anddischarge pressures and monitoring these values in the control system.Typically, the suction side pressure is essentially constant, becausethe buffer tank serving as a source is at atmospheric pressure. Thedischarge back pressure is set to a minimum level, corresponding to thedesired maximum flow rate, and the pressure is monitored by means ofpressure sensors.

In order to bring the liquid in the return circuit to a temperaturecorresponding to the minimum temperature of the feed, a heat exchangerand appropriate temperature sensors are provided in the return line. Theheat exchanger is dimensioned correspondingly, and minimum limits forcooling water flow and temperature are set in the control system.

A sterilization apparatus according to the present invention comprises apump capable of a certain maximum flow rate, preferably a displacementpump, by means of which a liquid to be sterilized is conducted through aheating unit at a constant rate. Following the heating unit, asterilization zone is arranged which is provided with temperaturemeasurement at least at the outlet end. Since the capacity of the pumpcan be kept constant, it can be ensured that the residence time in thesterilization zone is sufficient to achieve the desired sterilizationlevel. In case the residence time is insufficient, the flow leaving thesterilization zone is conducted through the return circuit back to theheating unit inlet Because the temperature measurement is arrangeddownstream from the heating unit, it is ensured that the residence timeis sufficient.

The return circuit is provided with cooling equipment. When the returncircuit is cooled essentially to the minimum temperature occurring inthe liquid in the storage tank, it can be ascertained that thetemperature of the input flow of the heating unit is not lower than theminimum level required by the heating means.

Preferably, the apparatus according to the invention comprises anarrangement of serial valves arranged after the sterilization zone, bymeans of which valve arrangement it is ensured that no insufficientlysterilized liquid, which has passed the sterilization zone, can flowoutside the apparatus, not even if there is a leakage in a distributingvalve.

An advantageous embodiment of the invention is described below withreference to the accompanying drawing.

The FIGURE shows a biowaste treatment apparatus according to theinvention. The main components provided in the main line of thetreatment apparatus in the flow direction of a biowaste-containingliquid are a storage tank 20, a heating unit 30 and a discharge coolingunit 40. In addition, the treatment apparatus comprises a circulationcircuit provided with a circuit cooling unit 50 and connected to themain line, by which circulation circuit the biowaste-containing liquidcan be circulated through heating unit 30.

Preferably, the biowaste water is conducted into the storage tankthrough a solid matter separating unit, which is not shown in thefigure. Storage tank 20 is provided with a mixer 21, and a driving motor22 connected thereto, by which the biowaste water in the storage tank 20is mixed to prevent sedimentation in the storage tank 20. The storagetank 20 is also provided with a level measurement L.

From the storage tank 20 the biowaste water is conducted into heatingunit 30 through an inlet valve 13 of the main line by means of aconstant capacity feed pump 31. In this embodiment, heating unit 30consists of a heat exchanger, in which steam is used as a heat source.After the heating unit, a sterilization zone 32 is arranged, whichherein is provided with two temperature measurements T1 and T2. Themeasurement of the outlet end 12 is essential, because at that point thelowest temperature occurs.

In continuous operation, the sterilized and deactivated biowaste wateris conducted from heating unit 30 into a discharge cooling unit 40 viavalve group 14 and 15 which forms a barrier site. From discharge coolingunit 40, the deactivated biowaste water is conducted through main linedischarge valve 17 to a sewer system at point A2. In this embodiment,discharge cooling unit 40 is a heat exchanger using water as a coolingmedium.

In addition to the above-described main line of the treatment apparatus,the apparatus comprises a return circuit beginning at the barrier siteof the main line between sterilization zone 32 and discharge coolingunit 40 and ending at the suction inlet of feed pump 31. In front of thefirst valve 14 of the barrier site of the main line, a first branch isarranged to the first parallel inlet valve 53, and between the firstvalve 14 and the second valve 15 of the barrier site of the main line, asecond outer branch is arranged to a second parallel inlet valve 54.Said serial valves 14 and 15 of the main line and said parallel valves53, 54 of the circulation circuit together form a barrier site. Aftersaid parallel valves 53, 54, the inner branch and outer branch arejoined together, whereafter the joined line of the circulation circuitleads to a circuit cooling unit 50 provided in the circulation circuit.After circuit cooling unit 50, the circulation circuit is closed viacircuit discharge valve 55 to a point between main line inlet valve 13and main line feed pump 31. The tightness of the first serial valve 14can be controlled by means of pressure measurement P coupled to the linebetween first serial valve 14 and second serial valve 15.

By means of said main line serial valves 14, 15 and said two parallelbranches of the circulation circuit, it is ensured that dischargecooling unit 40 and the subsequent zones cannot under any conditions becontaminated.

The cooling water needed in cooling units 40, 50 is brought into circuitcooling unit 50 through cooling water inlet valve 51 at point D1. Thecooling water circulated in circuit cooling unit 50 is conducted furtherto discharge cooling unit 40. The cooling water circulated in dischargecooling unit 40 is discharged via cooling water discharge valve 52 atpoint D2.

The steam needed in heating unit 30 is fed from point E1 30 throughfirst main steam line inlet valve 71 and second inlet valve 72 intoheating unit 30. The condensate formed in heating unit 30 is dischargedat point E2.

The startup of the apparatus is carried out by self-testing effected bya control system. Thereafter, biowaste is fed into tank 20, and thecirculation of the biowaste is started in beating unit 30 at constantspeed by means of the circuit, while the temperature of heating unit 30is raised to the desired level. The circulating water is cooled incircuit cooling unit storage tank. In this way it is ensured that theload of the heating unit does not exceed its capacity at the beginningof the continuous process.

In the above described startup stage, the integrity of valve 14 is alsotested by means of pressure measurement P. If no pressure rise is foundby pressure measurement P, valve 14 operates in the desired manner. Ifas a result of defective tightness of the valve, a pressure rise occurs,it is possible to safely conduct the flow further into the returncircuit through the outer branch and valve 53. Thereby valves 14, 15,53, 54 can be sterilized for maintenance by raising the temperature ofthe whole circuit to a sufficient level for a sufficient time.

When the measurement of the temperature of the outlet end of thesterilization zone shows that the temperature in the sterilization zoneis maintained at a sufficient level relative to the constant flow rate,the circuit can be interrupted by closing valve 53 and opening valve 14,and the deactivated biowaste water can be conducted through dischargecooling means 40 to the sewer system at point A2.

Preferably, the sterilization apparatus shown in the FIGURE iscontrolled by means of a control system or a computer. Information onthe state and operation of all components shown in the FIGURE are fedinto the control system, and on the basis of this information the statusof the components as well as of the whole apparatus can be shown on adisplay. In the FIGURE, only the components necessary for understandingthe invention are shown, and all other components, e.g. those relatingto various measurements, have been left out.

1. A method of sterilizing a biological waste, comprising the steps of: a) initiating heating of a heating unit (20); b) conveying a waste liquid having an initial temperature from a source through said heating unit (20) at a predetermined rate to heat said waste liquid; c) monitoring a temperature (T2) of said waste liquid after said waste liquid passes through said heating unit (20); d) comparing said temperature (T2) to a desired sterilization temperature, said temperature (T2) is less than the desired sterilization temperature; e) cooling said waste liquid to said initial temperature; f) returning said waste liquid at said initial temperature back through said heating unit (20) at said predetermined rate; and g) repeating steps c, d, e, and f until said temperature (T2) is greater than or equal to said sterilization temperature.
 2. A method of sterilizing a biological waste liquid as defined in claim 1, further comprising the step of: h) continuously conveying waste fluid from said source through said heating unit (20) and a first cooling unit (40) to be discharged therefrom when said temperature (T2) is greater than or equal to said sterilization temperature.
 3. A method of sterilizing a biological waste liquid as defined in claim 1, wherein the step e) of cooling includes the step of: conveying said waste liquid through a second cooling unit (50).
 4. An apparatus for the sterilization of a biological waste liquid, comprising: a storage tank (20) for storing a waste liquid, wherein said waste liquid has an initial temperature; a heating unit (30) for heating said waste liquid disposed downstream of said storage tank (20); a sterilization zone (32) disposed downstream of said heating unit (30); a pump (31) for conveying a waste liquid from said storage tank (20) along a first flow path through said heating unit (30) and through said sterilization zone (32); a first serial valve (14) and a second serial valve (15) disposed in series after said sterilization zone (32) within said first flow path; a second flow path beginning downstream of said sterilization zone (32) directing said waste liquid through said heating unit (30) and through said sterilization zone (32); at least one means for measuring a temperature of said waste liquid after said heating unit (30); a cooling unit (50) disposed within said second flow path for cooling said waste liquid to a predetermined temperature; a first branch conduit having a first end fluidly connected to said first flow path between said sterilization zone (32) and said first serial valve (14) and a second end fluidly connected to said cooling unit (50); and a first parallel valve disposed within said first branch conduit between said first flow path and said cooling unit (50).
 5. An apparatus for the sterilization of a biological waste liquid, comprising: a storage tank (20) for storing a waste liquid, wherein said waste liquid has an initial temperature; a heating unit (30) for heating said waste liquid disposed downstream of said storage tank (20); a sterilization zone (32) disposed downstream of said heating unit (30); a pump (31) for conveying a waste liquid from said storage tank (20) along a first flow path through said heating unit (30) and through said sterilization zone (32); a first serial valve (14) and a second serial valve (15) disposed in series after said sterilization zone (32) within said first flow path; a second flow path beginning downstream of said sterilization zone (32) directing said waste liquid through said heating unit (30) and through said sterilization zone (32); at least one means for measuring a temperature of said waste liquid after said heating unit (30); a cooling unit (50) disposed within said second flow path for cooling said waste liquid to a predetermined temperature; a second branch conduit having a first end fluidly connected to said first flow path between said first serial valve (14) and said second serial valve (15) and a second end fluidly connected to said cooling unit (50); and a second parallel valve disposed within said second branch conduit between said first flow path and said cooling unit (50).
 6. An apparatus for the sterilization of a biological waste liquid as defined in claims 4 or 5, wherein said predetermined temperature is said initial temperature.
 7. An apparatus for the sterilization of a biological waste liquid as defined in claims 4 or 5, wherein said heating unit (30) is a heat exchanger for transferring heat from steam to said waste liquid.
 8. An apparatus for the sterilization of a biological waste liquid as defined in claims 4 or 5, wherein means for determining a pressure of said waste liquid is disposed between said first serial valve (14) and said second serial valve (15). 